Polymer/Nano-Inorganic Composite Proton Exchange Membranes for Direct Methanol Fuel Cell Application Hongze Luo Submitted in fulfillment of the requirements for the degree of MSc in Chemistry in the Department of Chemistry, University of the Western Cape Supervisor: Prof. Vladimir M. Linkov Dr. Ji Shan April 2005
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Polymer/Nano-Inorganic Composite Proton Exchange
Membranes for Direct Methanol Fuel Cell Application
Hongze Luo
Submitted in fulfillment of the requirements for the degree of MSc in Chemistry
in the Department of Chemistry, University of the Western Cape
Supervisor: Prof. Vladimir M. Linkov
Dr. Ji Shan
April 2005
Keywords
KEYWORDS
Fuel Cell
Direct Methanol Fuel Cell (DMFC)
Proton exchange membrane (PEM)
Proton conductivity
Water uptake
Phosphorized zirconium oxide nano-particles (ZP)
Sulfonated poly ether ether ketone (SPEEK)
Methanol permeability (methanol cross-over)
Degree of sulfonation (DS)
Membrane-electrode assembly (MEA)
ii
Declaration
Declaration
I declare that Polymer/Nano-Inorganic Composite Proton Exchange Membranes for
Direct Methanol Fuel Cell Application is my own work, and that it has not been
submitted for any degree or examination in any other university, and that all sources I
have used or quoted have been indicated and acknowledged by complete references.
Hongze Luo October, 2004
Signed: …………………………………
iii
Acknowledgements
Acknowledgements
I would like to express my sincere gratitude to all those who helped me with my
study at UWC. I wish to thank the following people:
Prof. Vladimir Linkov for affording me the opportunity to be part of his research
group;
Supervisor Dr. Ji Shan, for his friendship, guidance throughout and constant
advice, and most especially for his assistance with the many technical aspects of
membrane technology;
Dr. G. Vaivars, for his guidance throughout and especially his assistance with ZP;
Dr. Z. Wang, for his assistance with the DMFC testing and constant support;
Dr. Y. Liu for her assistance with the FTIR measurements;
Prof. Key, staff and members of the chemistry department of the University of
Western Cape for their assistance;
L. Patrick, Ilie, Barbara Rodgers and Amanda Foster for their assistance in the
laboratory;
The Inorganic Porous Media Group, especially Mmalewane Modibedi and
Nicollete Hendricks for their encouragement;
Nolan and Basil in the Physics Department for their assistance with the SEM
measurements;
My parents and younger brother, for their patience, unending support, love and
understanding during the time of my studies.
iv
List of abbreviations
LIST OF ABBREVIATIONS
AFC Alkaline Fuel Cell DMAc N, N Dimethylacetamide DMFC Direct Methanol Fuel Cell DS Degree of Sulfonation FTIR Fourier Transform Infra-Red GC Gas Chromatograph HHV High Heat Value H2-PEMFC H2 Proton Exchange Membrane Fuel Cell I Current density [A/cm2] ICE Internal Combustion Engine IEC Ion Exchange Capacity l Membrane thickness [cm] MCFC Moltern Carbonate Fuel Cell MEA Membrane Electrode Assembly Nafion/ZP Nafion/ phosphorized zirconium oxide nano-particles P Methanol permeability [cm2/s] PAFC Phosphoric Acid Fuel Cell PBI Polybenzimidazole PEEK Poly Ether Ether Ketone PEM Proton Exchange Membrane PEMFC Proton Exchange Membrane Fuel Cell PSU Polysulfone PTFE Polytetrafluoroethylene R Resistance [Ω] S Surface area [cm2] SEM Scanning Electron Microscopy SOFC Solid Oxide Fuel Cell SPEEK Sulfonated Poly Ether Ether Ketone SPEEK/ZP Sulfonated Poly Ether Ether Ketone/ Phosphorized Zirconium oxide
nano-particles SPI Sulfonated Polyimide TEM Transmission Electron Microscopy TEOS Tetraethyl Orthosilicate TGA Thermal Gravimetric Analysis V Voltage [V] XRD X-Ray Diffraction ZP Phosphorized Zirconium oxide nano-particles σ Proton conductivity [S/cm]
v
Abstract
Abstract
The proton exchange membrane (PEM) is one key component of direct methanol
fuel cells (DMFCs), which has double functions of conducting protons, separating
fuels and oxidant. At present, the performance and price of sulfonic acid PEMs used
in DMFCs are deeply concerned. In order to reduce membrane’s cost and improve
performance of Nafion membrane, three different kinds of membranes have been
studied in this thesis.
Chapter 3, Chapter 4 and Chapter 5 present those three different but related types
of membranes, respectively.
In Chapter 3, sulfonated poly(ether ether ketone) (SPEEK) was synthesized by
sulfonating poly(ether ether ketone ) with 98% sulfuric acid.
SPEEK membranes possess good thermal stability and mechanical properties, low
methanol permeability (P = 4.00×10−9 cm2/s at DS = 0.30) and the proton
conductivity (σ = 2.5 × 10−2 S/cm at DS = 0.82). The proton conductivity of the
SPEEK membranes, water uptake and methanol permeability were increased with
increasing DS and temperature.
In Chapter 4, SPEEK/phosphorized zirconium oxide nano-particles (ZP) composite
membranes were prepared by incorporating various ratios of ZP into SPEEK.
SPEEK/ZP membranes showed many improved properties. Key amongst these are
vi
Abstract
increased conductivity, reduced water uptake and the 28% methanol permeability
reduction of a membrane with 5 wt% of ZP compared with that of SPEEK membrane,
it is 12 times lower than that of Nafion® 117. A DMFC testing result showed a
promising performance. SPEEK/ZP composite membrane with low incorporated ZP
content is considered for DMFC application.
In Chapter5, a series of Nafion/ZP composite membranes were also prepared and
investigated to overcome the shortcomings of Nafion®. The incorporated ZP
increased the proton conductivity and maintained at high temperature. The
conductivity of the composite membranes exceeded S/cm at room
temperature and reached a value of S/cm at 100
2102.2 −×
2103.8 −× oC for Nafion/ZP (70% ZP)
membrane. The composite membrane with low amount of ZP incorporated shown
lower methanol crossover comparing to Nafion. Nafion/ZP membrane can be used as
candidate proton exchange membranes for the high temperature operation of the
DMFCs.
vii
Table of contents
TABLE OF CONTENTS TATLE PAGE..................................................................................................................viii KEYWORDS ...................................................................................................................... iiDECLARATION .............................................................................................................viii ACKNOWLEDGEMENTS .............................................................................................viii LIST OF ABBREVIATIONS..........................................................................................viii ABSTRACT....................................................................................................................... viTABLE OF CONTENTS.................................................................................................viii LIST OF FIGURES .........................................................................................................viii LIST OF TABLES ...........................................................................................................viii CHAPTER 1 Introduction 1.1 Background ................................................................................................................... 1 1.2 Objectives...................................................................................................................... 2 1.3 Steps towards attaining solutions .................................................................................. 3 CHAPTER 2 Literature review 2.1. Background of Energy resources ................................................................................. 5 2.2. History of Fuel Cell Technology.................................................................................. 6 2.3. Advantages of Fuel Cells compared to conventional technologies ............................. 7 2.4. Types of fuel cells ........................................................................................................ 8 2.5. Proton Exchange Membrane Fuel Cells (PEMFC).................................................... 10
2.5.2.1. Advantages and comparison of DMFC with H2-PEMFC........................ 12 2.5.2.2. Principle of Direct Methanol Fuel Cell .................................................... 13 2.5.2.3. Components of a Direct Methanol Fuel Cell ........................................... 14 2.5.2.4. Membrane electrode assembly (MEA) .................................................... 15
2.8. The significance of the review ................................................................................... 38 CHAPTER 3 Preparation and Characterization of SPEEK membranes 3.1. Introduction................................................................................................................ 39 3.2. Experimental .............................................................................................................. 41
3.2.1. Chemical materials............................................................................................ 41 3.2.2. Sulfonating PEEK ............................................................................................. 41 3.2.3. Preparation of SPEEK membranes ................................................................... 42 3.2.4. Characterization of the SPEEK membranes ................................................... 43
3.2.4.1. FTIR study ............................................................................................... 43 3.2.4.2. Thermal-gravimetric analysis (TGA)....................................................... 43 3.2.4.3. Water uptake ............................................................................................ 44 3.2.4.4. Proton conductivity .................................................................................. 45 3.2.4.5. Measurement of methanol permeability................................................... 46
3.3. Results and discussions .............................................................................................. 49 3.3.1. Thermal property and degree of sulfonation..................................................... 49 3.3.2. Solubility of SPEEK ......................................................................................... 53 3.3.3. FTIR study ........................................................................................................ 53 3.3.4. Methanol permeability (methanol cross-over) ................................................ 56 3.3.5. Water uptake ..................................................................................................... 57 3.3.6. Proton conductivity ........................................................................................... 58
3.4. Summary .................................................................................................................... 61 CHAPTER 4 Preparation and Characterization of SPEEK/ZP composite membranes 4.1. Introduction................................................................................................................ 62 4.2. Experimental .............................................................................................................. 64
4.2.1. Chemical materials............................................................................................ 64 4.2.2. Preparation of composite membranes ............................................................... 64
4.2.2.1. Preparation of ZP ................................................................................... 64 4.2.2.2. Pre-treatment of SPEEK .......................................................................... 65
stability) ................................................................................................. 69 4.2.3.8. Measurement of proton conductivity ....................................................... 69
4.2.4. DMFC testing.................................................................................................... 69 4.2.4.1. Preparation of the membrane-electrode assembly (MEA) ..................... 69 4.2.4.2. Assembly of single cell and test............................................................... 70
4.4. Summary .................................................................................................................... 87 CHAPTER 5 Preparation and Characterization of Nafion/ZP composite membrane 5.1. Introduction................................................................................................................ 89 5.2. Experimental .............................................................................................................. 91
5.2.1. Preparition of ZP............................................................................................... 91 5.2.2. Preparation of composite membranes ............................................................... 91 5.2.3. Characterization of the membranes................................................................... 92
Chapter 2 Figure 2.1: Basic description of a DMFC operation ...................................................... 14 Figure 2.2: A view of a DMFC stack and an exploded view of a single cell ................ 15 Figure 2.3: Chemical structure of Nafion® membrane .................................................. 19 Figure 2.4: Chemical structure of Dow ionomer membrane.......................................... 20 Figure 2.5: Chemical structure of sulfonated polyetherketone ...................................... 25 Figure 2.6: Poly[2,20-(m-phenylene)-5,50-bibenzimidazole], PBI ............................... 28 Figure 2.7: Cluster-network model for Nafion® membrane .......................................... 33 Figure 2.8: A: Three region structural model for Nafion®
B: Schematic representation of microstructure of Nafion® ......................... 34 Figure 2.9: Schematic of membrane showing the interconnecting channel swollen ..... 35 Figure 2.10: Schematic of a membrane showing the collapsed interconnecting
channel .......................................................................................................... 35 Figure 2.11: Schematic representation of the microstructure of Nafion® and a
sulfonated polyetherketone membrane ......................................................... 37 Chapter 3 Figure 3.1: Schematic representation of the cell for measuring conductivity................ 45 Figure 3.2: Schematic diagram of methanol permeability measurement....................... 47 Figure 3.3: Chemical structure of PEEK ....................................................................... 49 Figure 3.4: TGA curves of PEEK .................................................................................. 50 Figure 3.5: TGA curves of SPEEK................................................................................ 51 Figure 3.6: Degree of sulfonation (DS) of SPEEK with the sulfonation time............... 53 Figure 3.7: Comparative FTIR spectra of PEEK and sulfonated PEEK........................ 54 Figure 3.8: Structure and atom numbering of SPEEK................................................... 56 Figure 3.9: Influence of DS on methanol permeability.................................................. 57 Figure 3.10: Water uptake as a function of DS at room temperature and 80℃................ 58 Figure 3.11: Conductivity of the membranes with different DS....................................... 59 Chapter 4 Figure 4.1: The procedures of membrane-electrode assembly ...................................... 70 Figure 4.2: Lynntech endplates ...................................................................................... 71 Figure 4.3: Schematic overview of the experimental setup for DMFC testing ............. 71 Figure 4.4: FTIR of ZrO2 and ZP................................................................................... 73 Figure 4.5: FTIR of SPEEK, ZP and composite membrane .......................................... 73
xii
List of figures
Figure 4.6: The XRD patterns of nano-sized ZrO2 and ZP............................................ 74 Figure 4.7: Thermo-gravimetric curves of PEEK, SPEEK and SPEEK/ZP.................. 75 Figure 4.8: Water uptake of the membranes with different ZP content......................... 77 Figure 4.9: Effect of incorporated ZP content on methanol permeability ..................... 78 Figure 4.10: SEM micrographs of membranes ................................................................. 81 Figure 4.11: Cyclic voltammetry of SPEEK/ZP composite membrane............................ 82 Figure 4.12: Proton conductivity of SPEEK/ZP composite membranes as a function
of temperature ............................................................................................. 83 Figure 4.13: Effect of ZP content on proton conductivity at room temperature............... 84 Figure 4.14: Effect of ZP content on proton conductivity at100 ℃ ................................. 84 Figure 4.15: Discharge curve of a single DMFC .............................................................. 86 Figure 4.16: SEM cross-section of MEA.......................................................................... 87 Chapter 5 Figure 5.1: XRD patterns of ZrO2 and various Nafion/ZP composite membranes ....... 94 Figure 5.2: SEM of composite membranes.................................................................... 95 Figure 5.3: Water uptake of Nafion/ZP composite membranes as a function of
temperature.................................................................................................. 97 Figure 5.4: Conductivity of Nafion/ZP membranes at room temperature and 100%
humidity ...................................................................................................... 98 Figure 5.5: Effect of different ZP content on the conductivity as a function of
temperature.................................................................................................. 99 Figure 5.6: The methanol permeability of Nafion and Nafion/ZP composite
membranes ................................................................................................ 101 Figure 5.7: TEM of nano-sized ZrO2 and ZP............................................................... 103
xiii
List of tables
List of tables
Table 2.1: Types of Fuel Cells ............................................................................................ 9 Table 3.1: Chemical materials........................................................................................... 41 Table 3.2: HP 5890 parameters......................................................................................... 48 Table 3.3: The properties of SPEEK membranes ............................................................. 52 Table 4.1: Chemical materials........................................................................................... 64 Table 4.2: The specifications and working parameters of XRD....................................... 67 Table 4.3: Operating parameters of SEM ......................................................................... 68 Table 4.4: present properties of the membranes ............................................................... 77
xiv
CHAPTER 1 Introduction
1.1. Background
Fuel cells are electrochemical energy converters, transforming chemical energy
directly into electricity. Due to their many benefits, they are forming an attractive new
technology of electricity generation. In the next few years, strides in fuel cell
technology will forever change our concept of alternative energy systems and will
become the driver of the next growth wave of the world’s economy [1]. As well as
offering a high theoretical efficiency, especially at low temperatures, fuel cells emit
low or zero levels of pollutants. They can run on a wide range of fuels –from the
gaseous, such as hydrogen and natural gas to the liquid fuels such as methanol and
gasoline [2].
DMFCs use methanol as the fuel. Methanol is a low-cost liquid fuel having high
electrochemical activity. It has a high energy density with respect to its volume,
which allows for greater efficiency over traditional combustion engines. Presently,
DMFCs are becoming very attractive for transportation and portable applications as
they offer important advantages such as elimination of fuel reforming, ease of
refueling, and simplified system design [3].
1
Introduction Chapter 1
Proton exchange membranes play the major role in DMFCs. They effect the
performance and the cost of DMFCs. Since, the proton exchange membranes based
on perfluorinated polymer, such as Nafion® membranes, exhibit high proton
conductivity and stability [4], they are widely used as the electrolyte.
However, the high methanol permeability (crossover) is the major drawback of
Nafion®. The methanol crossover or diffusion across the polymer exchange
membrane from the anode to the cathode in the DMFC causes loss of fuel, reduced
cathode voltage and excess thermal load in the cell. It strictly limits the performance
of the DMFC [5 - 9]. Furthermore, the high price of fluorine-based Nafion® is the major
factor influencing the cost of the DMFC system. The commercialization of DMFCs is
thus limited by the high cost of available perfluorinated membranes [10, 11]. Therefore,
an important task is the development of cheap membranes with low methanol
crossover.
1.2. Objectives
The main objective of this project is to develop an inexpensive proton exchange
membrane with low methanol crossover and high conductivity. The main focus of
this study includes:
2
Introduction Chapter 1
1. A search for cheaper membrane non-perfluorinated materials.
2. Preparation of membranes, and developing effective membrane synthesis
methods.
3. Modification of the membrane structure to improve the properties of the
membrane.
4. Characterizing the properties of the prepared membrane using different
techniques.
5. Assembling the single cell of a DMFC using synthesized membrane, and
evaluating its performance and its commercial potential.
6. To modify the existing Nafion® membrane to overcome its shortcomings.
1.3. Steps towards attaining solutions
1. According to the literature review, the basic starting material identified is SPEEK.
2. Sulfonating PEEK to various levels and casting the membranes with the prepared
SPEEK. Chapter 3 introduces a detailed preparation of SPEEK, preparation of
SPEEK membranes, their characterizations, results and discussions.
3. Incorporating phosphorized zirconium oxide nano-particles (which are nano-sized
inorganic proton conductors) into SPEEK for improving the properties of SPEEK
membranes. Chapter 4 presents a detailed synthesis of SPEEK/ZP membranes
3
Introduction Chapter 1
including preparation of ZP, characterization of SPEEK/ZP composite
membranes, the testing of a DMFC using prepared SPEEK/ZP membrane, results
and discussion.
4. Incorporating various ratio of ZP into Nafion to prepare a series of Nafion/ZP
composite membranes which are expected to overcome the technical
shortcomings of Nafion membrane. Chapter 5 gives a detailed preparation of
Nafion/ZP composite membranes, characterization of Nafion/ZP membrane,
results and discussions.
4
Literature review Chapter 2
CHAPTER 2 Literature Review
2.1. Background of Energy resources
The increase in energy usage (especially coal, crude oil, natural gas) for every
citizen in the world has increased rapidly. Fossil energy resources will be used up
within just a few generations if present usage levels are sustained and the availability
of energy resources globally is becoming a key issue for the future.
Pollution arising form the current techniques of energy use is a major problem and
newer more efficient and clean techniques are required.
Fuel cells have gained popular recognition and are under serious consideration as
an economically and technically viable power source. They are considered a prime
candidate for the future — being clean, quiet, and efficient.
According to President George Bush, speaking from the White House Lawn,
February 25, 2002, “We happen to believe that fuel cells are the wave of the future…
we need to have a focused effort to bring fuel cells to market, and that’s exactly what
my administration is dedicated to do.” [12]
5
Literature review Chapter 2
2.2. History of Fuel Cell Technology
The origin of fuel cell technology in 1839, is credited to William Robert Grove
(1811-1896) who was a British jurist and amateur physicist [13, 14]. Ludwig Mond
(1839-1909) with assistant Carl Langer conducted experiments with a hydrogen fuel
cell in 1888 that produced 6 amps per square foot at 0.73 volts [15]. Francis Thomas
Bacon invented the first alkaline fuel cell (AFC) in 1932 [16]. 27 years later, he made a
5 kw fuel cell for practical application.
From 1839, it took 120 years until NASA demonstrated some potential applications
in providing power during space flight. During that time, the first PEMFC was
invented and developed [17, 18]. As a result of these successes, industry recognized the
commercial potential of fuel cells in the 1960s, but encountered technical barriers and
high investment costs. Since 1984, the Office of Transportation Technologies at the
U.S. Department of Energy has been supporting research and development of fuel cell
technology [19]. Hundreds of companies around the world are working towards
making fuel cell technology pay off. In 1993, Ballard Corporation made the first fuel
cell car in Canada. Just as in the commercialization of the electric light bulb nearly
one hundred years ago, today’s companies are being driven by technical, economic,
and social forces such as high performance characteristics, reliability, durability, low
cost, and environmental benefits [20].
6
Literature review Chapter 2
2.3. Advantages of Fuel Cells compared to conventional technologies
Fuel cells are not Carnot cycle (thermal energy based) engines [21]. Since the fuel is
converted directly to electricity, a fuel cell has the potential to operate at much higher
efficiencies than in conventional energy conversion processes, thereby extracting
more electricity from the same amount of fuel, while providing the heat of
condensation of the water vapour in the products. Fuel cells have low emission
profiles. If a hydrogen fuel is used, the only waste product is water. Fuel cells are
mechanically ideal because these devices have no moving parts thereby making them
quiet and reliable sources of power.
In principle, a fuel cell operates like a battery [22, 23]. Unlike a battery, a fuel cell
does not run down or require recharging. A fuel cell will be able to continually
generate energy as long as fuel and the oxidant are provided to the cell. This is
distinctly different from typical batteries, which are merely energy storage devices.
Since it is a storage appliance, the battery is dead (or discharged) when the stored
reactants are exhausted. The fuel for fuel cells is stored external to the actual device,
and therefore, can not become internally depleted [24, 25].
7
Literature review Chapter 2
2.4. Types of fuel cells
Fuel cell types are generally classified by different electrolyte material. The
electrolyte is the substance between the positive and negative electrode, acting as the
conductor (but it does not conduct electrons) for the ion exchange that produces
electrical current [26]. There are five kinds of fuel cell undergoing study (Table 2.1),
development and demonstration, in various stages of commercial availability. These
five types of fuel cell are significantly different from each other in many respects that
the key distinguishing feature is the electrolyte material. All fuel cells have the same
basic operating principle.
8
Literature review Chapter 2
Table 2.1: Types of Fuel Cells [27, 28]
Type
Alkaline Fuel Cell (AFC)
Molten Carbonate fuel cells (MCFC)
Phosphoric Acid Fuel Cells (PAFC)
Solid Oxide fuel cells (SOFC)
Proton Exchange Membrane Fuel Cells (PEMFC)
Type of electrolyte
Typically aqueous KOH solution
Typically, molten Li2CO3/K2CO3 eutectics
H3PO4
solutions Stabilized ceramic matrix with free oxide ions
Due to their low-cost, higher conductivity, suitable water uptake and low methanol
permeability, SPEEK/ZP composite membranes are considered for use in DMFCs as
alternatives to Nafion®.
At last in chapter 5, a series of Nafion/ZP composite membranes were prepared by
incorporating ZP in Nafion®. These membranes exhibited a rather high conductivity
of S/cm at room temperature and reached a maximum S/cm at
100 ℃ for Nafion/ZP(70wt%) membrane. The composite membrane with low
incorporated ZP content showed lower methanol permeability compared to Nafion
2109.2 −× 2101.9 −×
®.
Increased ZP content reduced the water retention, but enhanced the proton
conductivity, especially at high temperature. The membranes are easy to prepare.
Their high proton conductivity and maintained at high temperature, reduced methanol
permeability compared to Nafion® qualify these composite membranes for
consideration for use in DMFCs.
The most frequently used analytical methods to characterize membranes and basic
106
Conclusions and recommendations Chapter 6
materials are TGA, SEM, FTIR etc. These methods provide information about the
physical and chemical properties.
6.2. Recommendations
We need to improve the quality of the interface between the SPEEK/ZP composite
membrane and the electrodes. The performance of the DMFC using prepared
SPEEK/ZP composite membrane can be improved if this problem is solved. The
development of more suitable electrodes and electrode-membrane assemblies is in
progress.
There is a need to test the lifetime of the composite membranes in actual DMFC
application. Further modification of the membranes via appropriate methods might be
needed in order to enhance the stability and proton conductivity of the membrane.
107
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